1. Field of the Invention
This invention relates to the heating of articles to elevated temperatures in a condensation heat transfer facility. In particular, the invention is directed to heating such articles that are heat sensitive and can remain above predetermined temperatures for only a limited time.
2. Description of the Prior Art
It is well known to heat articles to elevated temperatures in a condensation heat transfer facility. Such a facility is fully described in U.S. Pat. No. 3,904,102 to T. Y. Chu et al. which is assigned to the instant assignee. That facility contains a body of hot saturated vapor of a boiling primary liquid which condenses on an article to be heated to the elevated temperature, transferring its latent heat of vaporization thereto. Additionally, the Chu et al. patent describes the use of a secondary body of vapor disposed above the body of relatively expensive hot primary vapor to prevent losses thereof to the atmosphere. This type of facility has been found to be extremely effective in heating a variety of articles for purposes of soldering, brazing, fusing, curing, cooking, testing, etc.
In particular, such a condensation heating facility has been used for reflow soldering operations wherein electrical components are soldered to printed wiring boards or the like. In most of these applications the length of time that the components are in the hot saturated primary vapors is not critical.
However, problems arise when external leads are to be soldered to metallized ceramic substrates in the condensation heat transfer facility. Bonding pad areas on the substrate are formed by deposition of a plurality of thin metallic layers to provide good adhesion to the ceramic as well as a low resistance electrical conductive path. A final layer of gold is deposited on the thin metallic layers to protect the layers and provide a reliable electrical contact. Solder subsequently is applied and reflowed to connect the external leads to the bonding pads.
It has been determined that such metallized substrates and external leads should be heated to an elevated temperature of approximately 50.degree. F. higher than the solder liquidus temperature. Heating to such an elevated temperature is necessary to insure adequate solder reflow required for acceptable bonds. However, when the metallized substrates are raised to such an elevated temperature, the cool-down time is substantially increased and the bond strength of the leads to the substrate deteriorates rapidly. It is believed that such prolonged time above the solder liquidus temperature undesirably causes substantial alloying of both the gold and the underlying thin metallic films with solder, forming intermetallics, resulting in weak or defective bonds.
Once the substrate and the external leads have been heated to the elevated temperature and then moved into the lower temperature secondary vapors, the cooling rate is relatively slow and the substrate temperature remains above the solder liquidus temperature for too long a period resulting in the aforementioned weak bonds. Accordingly, the metallized ceramic substrate and the external leads to be soldered thereto can remain in the condensation heat transfer facility for only a limited time after being heated to the elevated temperature. However, the substrate with the leads bonded thereto must be removed slowly from the facility through the secondary vapor so as to: (1) minimize disruption of the primary-secondary vapor interface, (2) permit drainage of the condensed vapors from the substrate, and (3) lessen drag-out losses of the primary vapors.